expose openvswitch GRE tunnel_id via provider API

expose openvswitch GRE tunnel_id via provider API

The provider:vlan_id extended attribute is renamed to
provider:segmentation_id, and the openvswitch plugin returns the
tunnel_id of GRE networks as this attribute.

Fixes bug 1044375. Final patch in series that fixes bug 1037341 and
bug 1035492.

Detailed changes:

- Rename provider:vlan_id to provider:segmentation_id.
- Use consts for providernet extension's attribute names
- Remove validation clause from provider:segmentation_id resource attribute map data.
- Validate provider:segmentation_id in linuxbridge and openvswitch plugins.
- Rename physical_id to segmentation_id in the openvswitch schema, plugin, and agent.

Change-Id: Ic023b30b6e3275955bcb8a09c7936035dacf3f87